There are a number of different considerations for manufacturing a resin suitable for use in rotomolding manufacture. The resin needs to be: capable of production at commercially acceptable rates of production; suitable for use in the rotomolding process (e.g., for example, having a suitable sintering temperature and a suitable cooling rate to be removed from the mold) and finally must have suitable properties for the end use application. One important property that is sought is environmental stress cracking. The resin should not develop cracks due to exposure to chemicals, sunlight, etc. in applications such as tank sprayers for agricultural use, cisterns, and smaller rotomolded parts.
U.S. Pat. Nos. 5,382,630, and 5,382,631, issued Jan. 17, 1995 to Stehling, assigned to Exxon, teach bimodal resins having superior physical properties. The patent requires that the blend have a two or more components, each having a polydispersity (Mw/Mn) of less than 3 and the blend having a polydispersity greater than 3 and no component in the blend having a relatively higher molecular weight and a lower comonomer content (e.g., the comonomer incorporation is reverse). The reference does not suggest improved ESCR.
U.S. Pat. No. 6,969,741, issued Nov. 29, 2005 to Lustiger et al., assigned to ExxonMobil, teaches a blend of polyethylenes suitable for rotomolding. The patent teaches the difference in the density of each component is not less than 0.030 g/cc. The difference in the densities of the component polymers in the present composition is less than 0.030 g/cc.
U.S. Pat. No. 7,230,054, issued Jun. 12, 2007 to Mavridis et al., assigned to Equistar, teaches a blend of a low density relatively higher molecular weight component and a high density relatively lower molecular weight component. The resin blends appear to be made in a slurry process suing Ziegler Natta catalysts or modified Ziegler Natta catalysts. The blends of the present invention are made using a solution polymerization process and a catalyst containing a phosphinimine ligand.
WO 2011/025742A1, published Mar. 3, 2011 in the name of Davis, assigned to Dow Global Technologies Inc., teaches a polymer blend having a high ESCR. The blend has a residual unsaturation of less than 0.06 per 1000 carbon atoms. The blends of the present invention have a residual unsaturation of greater than 0.06 per 1000 carbon atoms.
U.S. Pat. No. 8,067,518 B2, published Nov. 29, 2011 in the names of Davey et al., assigned to Univation, teaches a polymer made in gas phase using a very specific catalyst has enhanced ESCR properties and processability. The polymers produced in accordance with the disclosure do not appear to be bimodal.
The present invention seeks to provide a novel bimodal polymer having excellent ESCR.